Depth-dependent variations of sedimentary dissolved organic matter composition in a eutrophic lake: Implications for lake restoration

New insights into pollution source analysis using receptor models: Effects of interaction between heavy metals and DOM on source identification and apportionment in rivers across industrial city

To effectively control pollution and protect the ecosystem, it is essential to accurately analyze the potential pollution sources of heavy metals (HMs) in rivers. However, the traditional source apportionment methods based on HMs disregard the interaction between HMs and dissolved organic matter (DOM). In this study, data of HMs and DOM was combined for tracing sources and assessing the effect of interaction between HMs and DOM on source apportionment in urbanized rivers that cross urban (URR), industrial (INR), and rural (RUR) regions. Four types of fluorescent substances were extracted from DOM: tryptophan-like (TRLF), microbial byproduct (MB), fulvic-like (FLF), and humic-like (HLF) fluorescence substances. Anthropogenic activities (42.3 %), microbial products (21.5 %), and geogenic origin (23.7 %) were respectively recognized as the dominant source in URR, INR, and RUR. Additionally, significant correlations were obtained between HMs and high molecular mass DOM. There was no direct effect pathway obtained between HMs and sources and distributional characteristics of HMs are influenced by both economic and social factors. HMs have been found to indirectly affect source apportionment through interaction with DOM. This work provided a comprehensive understanding of the effects mechanism of the interaction of HMs and DOM on source identification and offered an effective method for tracing pollution sources.

Molecular weight-dependent differences in spectral properties and metal-binding behaviors of dissolved organic matter from different lakes

Dissolved organic matter (DOM) plays an important role in governing metal speciation and migration in aquatic systems. In this study, various DOM samples were collected from Lakes Erhai, Kokonor, and Chaka, and size-fractionated into high molecular weight (HMW, 1 kDa-0.7 μm) and low molecular weight (LMW, <1 kDa) fractions for measurements of dissolved organic carbon (DOC), spectral properties, and metal binding behaviors. Our results demonstrated that samples from Lake Chaka exhibited the highest DOC concentration and fluorescence indices but the lowest percentage of carbohydrates. Regardless of sampling locations, the HMW-DOM fractions contained higher abundances of aromatic DOM, carbohydrates and protein-like substances, but lower abundance of fulvic acid-like substances compared to those in the LMW fractions. Metal titration experiments coupled with the excitation-emission matrix (EEM)-parallel factor (PARAFAC) modeling revealed that the quenching of the PARAFAC-derived fluorescent components was more pronounced in the presence of Cu(II) compared to Pb(II). Humic-like components emerged as a superior model, exhibiting higher binding affinities for Cu(II) than protein-like substances, while the opposite trend was observed for Pb(II). In samples obtained from Lakes Erhai and Kokonor, the condition stability constants (Log KM) for the binding of both Cu(II) and Pb(II) with the HMW-DOM fraction were higher than those with the LMW-DOM fraction. Conversely, a contrasting trend was observed for Lake Chaka. This study highlighted the heterogeneity in spectral properties and metal-binding behaviors of natural DOMs, contributing to an improved understanding of the molecular interactions between DOM components and metal ions and their environmental fate in aquatic ecosystems.

Elucidating the Composition and Transformation of Dissolved Organic Matter at the Sediment-Water Interface Using High-Resolution Mass Spectrometry

The exchange and transformation of dissolved organic matter (DOM) at the sediment-water interface are crucial factors in regulating watershed biogeochemistry, with the molecular composition of DOM serving as a pivotal determinant in elucidating this process. High-resolution mass spectrometry (HRMS) is an effective tool for resolving the composition of DOM. By analyzing the compositional characteristics of DOM at the sediment-water interface under three different salinities at the same latitude region in northern China, the findings indicate certain variations in component characteristics of DOM between low-salinity inland waters and high-salinity seawaters, with the former exhibiting greater molecular diversity and higher molecular weights, whereas the latter displayed a higher saturation and bioavailability. Notably, the presence of more CHOS substances in the low-salinity inland waters underscores the transformation of the DOM influenced by terrestrial inputs and anthropogenic activities. Conversely, the presence of more CHO and CHNO substances in high-salinity seawater underscores the microbial effects. The chemical transformation process from overlying water to pore water to sediments was characterized by methylation, hydrogenation, decarboxylation, and reduction, as determined by calculating the relations between the H/C and O/C ratios of different compound types. These findings indicate that HRMS can yield more refined results in revealing the process of DOM at the sediment-water interface under different environments, which provides a more reliable basis for a deeper understanding of the source-sink mechanism of sediment organic matter.

Transformation of sedimentary dissolved organic matter in electrokinetic remediation catalogued by FT-ICR mass spectrometry

In electrokinetic remediation (EKR), the sedimentary dissolved organic matter (DOM) could impede remediation by scavenging reactive species and generating unintended byproducts. Yet its transformation and mechanisms remained largely unknown. This study conducted molecular-level characterization of the water-extractable DOM (WEOM) in EKR using negative-ion electrospray ionization coupled to 21 tesla Fourier transform ion cyclotron resonance mass spectrometry (21 T FT-ICR MS). The results suggested that ∼55 % of the ∼7,000 WEOM compounds identified were reactive, and EKR lowered their diversity, molecular weight distribution, and double-bond equivalent (DBE) through a combination of electrochemical and microbial redox reactions. Heteroatom-containing WEOM (CHON and CHOS) were abundant (∼ 35% of the total WEOM), with CHOS generally being more reactive than CHON. Low electric potential (1 V/cm) promoted the growth of dealkylation and desulfurization bacteria, and led to anodic CO2 mineralization, anodic cleavage of -SO and -SO3, and cathodic cleavage of -SH2; high electric potential (2 V/cm) only enriched desulfurization bacteria, and differently, led to anodic oxygenation and cathodic hydrogenation of unsaturated and phenolic compounds, in addition to cathodic cleavage of -SH2. The long-term impact of these changes on soil quality and nitrogen-sulfur-carbon flux may be need to studied to identify unknown risks and new applications of EKR.

The continuous increased stability of sediment dissolved organic matter implies ecosystem degradation of lakes in the cold and arid regions

The characteristics of lake dissolved organic matter (DOM) pool and lake ecosystem interact, and studying the responses between sediment DOM characteristics and lake ecosystem changes may shed light on the inherent connection between ecosystem evolution and carbon biogeochemical cycles. Lakes in cold and arid regions are sensitive to changes and accumulate large amounts of carbon as DOM, which may provide a window into more explicit relationships between ecosystem evolution and changes in sediment DOM characteristics in time dimension. However, considerable blind spots exist in the responses between the sediment DOM and ecosystem evolution on time scale and the underlying mechanisms. In this study, multiple approaches were combined to investigate the relationship between the variation trend of sediment DOM characteristics and the evolution of fragile lake ecosystems across three different lake ecosystems in cold and arid regions of China. A strong positive relationship between sediment DOM stabilities, especially humification, and ecosystem degradation was found, consistent for the three lakes. Ultra-high-resolution mass spectrometry and structural equation modeling revealed that the changes of ecosystems affected sediment DOM stability through direct pathways (0.24), such as the contents of terrestrial DOM in lake DOM pool, and indirect pathways, including algae-mediated (0.43) and salinity-mediated pathways (0.22), which all increased the contents of refractory DOM in the lake DOM pool and sediments. Based on the fact that DOM stability changes could act on the ecosystem in turn, a possible positive feedback mechanism between ecosystem degradation and increased DOM stability was further inferred. These results suggested that the continuous increased stability of sediment DOM in may implies ecosystem degradation of lakes in the cold and arid regions. This study provides a new perspective for recognizing ecosystem evolution through sediment DOM and improves the understanding of the interaction of lake ecosystem evolution and the biogeochemical cycle of DOM.

Depth-dependent variations of physicochemical properties of sedimentary dissolved organic matter and the influence on the elimination of typical pharmaceuticals

Sedimentary dissolved organic matter (DOM) could exert a significant influence on the transformation of trace organic contaminants. However, the variations of sedimentary DOM properties with depth and their impact on trace organic contaminants biodegradation remain unclear. In this study, the qualitative changes in DOM properties with depth were assessed using spectral techniques. Specifically, within the sediment range of 0-30 cm, humic acid and fulvic acid fractions exhibited higher degrees of humification and aromatization at 10-20 cm, while hydrophilic fractions showed higher degrees of humification and aromatization at 20-30 cm. Furthermore, electrochemical methods were employed to quantitatively assess the electron transfer capacity of sedimentary DOM at different depths, which displayed consistent variation trend with humification and aromatization degree. The high degree of humification and aromatization, along with strong electron-accepting capability of DOM, significantly enhanced the biodegradation rates of tetracycline and ritonavir. To gain deeper insights into the influence of molecular composition of DOM on its properties, two-dimensional gas chromatography-quadrupole mass spectrometry analysis revealed that quinones and phenolic hydroxyl compounds govern the redox reactivity of DOM. Simulated experiment of DOM-mediated biodegradation of typical pharmaceuticals confirmed the role of quinones and phenolic hydroxyl groups in the redox reactivity of DOM.

Molecular insight into the vertical migration and degradation of dissolved organic matter in riparian soil profiles

Due to the molecular complexity of dissolving organic matter (DOM), the vertical molecular distribution of riparian soil DOM (especially dissolved organic nitrogen (DON) and dissolved organic phosphorus (DOP)) in different land use types and their relationship with the bacterial community is still unclear. This study analyzed the spectral characteristics of riparian soil DOM from 0 to 100 cm in wild grassland, agricultural land, and bare land. The molecular distribution of DOM was revealed through Fourier-transform ion cyclotron resonance mass spectrometry (FT-ICR MS) and the specific relationship between DOM and bacterial community composition (BCC) was evaluated. The results showed that the DOM in the upper soil layer (0-40 cm) was mainly composed of recalcitrant macromolecular organics, while that in the lower layer (40-100 cm) was labile small molecular organics. In agricultural land, the total storage of DOM was lower than that in wild grassland, but with a higher abundance of recalcitrant organic carbon (lignin, etc.). At the same time, the bacterial community in agricultural land is shifting towards copiotrophs. In addition, the abundance of labile C degrading genes increases with nitrate as the main electron acceptor. However, sulfates are mainly used as electron acceptors in wild grasslands. Both DOP and DON were dominated by lignin and displayed higher chemical diversity in the upper soil. The bioavailability of DOP in three types of soil is higher than that of DON. DOM-BCC network analysis shows that the recalcitrant DON and DOP molecules in soil are positively correlated with phylum Actinobacteriota in agricultural land. These results emphasize that the DOM molecular characteristics were closely related to the function of the soil bacterial community.

The structural transformation reversibility of biogas slurry derived dissolved organic matter and its binding properties with norfloxacin under temperature fluctuation

The widespread use of biogas slurry could potentially raise the environmental risk of antibiotics. Dissolved organic matter (DOM), as the most active part of biogas slurry, was able to interact with antibiotics and play a crucial role in the structure and function of soil and aquatic ecosystems. The recent shifts in global climate patterns have garnered significant attention due to their substantial impact on temperature, thereby exerting a direct influence on the characteristics of DOM and subsequently on the environmental behavior of antibiotics. However, there is limited research concerning the impact of temperature on the binding of DOM and antibiotics. Thus, this study aimed to explore the temperature-dependent structural transformation and driving factors of biogas slurry-derived DOM (BSDOM). Additionally, the binding characteristics between BSDOM and the commonly used antibiotic norfloxacin (NOR) at different temperatures were studied by using multi spectroscopic methods and two-dimensional correlation spectroscopy (2D-COS) analysis. The results suggested that the temperature-dependent structural transformation of BSDOM was reversible, with a slight lag in the transition temperature under cooling (13 °C for heating and 17 °C for cooling). Heating promoted the conversion of protein-like to humic-like substances while cooling favored the decomposition of humic-like substances. BSDOM and NOR were static quenching, with oxygen-containing functional groups such as C-O and -OH playing an important role. Temperature influenced the order of binding, the activity of the protein fraction, and its associated functional groups. At temperatures of 25 °C and 40 °C, the fluorescent components were observed to exhibit consistent binding preferences, whereby the humic-like component demonstrated a greater affinity for NOR compared to the protein-like component. However, the functional group binding order exhibited an opposite trend. At 10 °C, a new protein-like component appeared and bound preferentially to NOR, when no C-O stretch corresponding to the amide was observed. The finding will contribute to a comprehensive understanding of the interaction mechanisms between DOM and antibiotics under climate change, as well as providing a theoretical basis to reduce the environmental risks of biogas slurry and antibiotics.

Effects of sediment dredging on freshwater system: a comprehensive review

As a common geo-engineering method to control internal load of nutrients and pollutants, sediment dredging has been used in many freshwater basins and has achieved certain effects. However, dredging can disturb water bodies and substrates and cause secondary pollution. It negatively affects the water environment system mainly from the following aspects. Dredging suddenly changes the hydrological conditions and many physical indicators of the water body, which will cause variations in water physicochemical properties. For example, changes in pH, dissolved oxygen, redox potential, transparency, and temperature can lead to a series of aquatic biological responses. On the other hand, sediment resuspension and deep-layer sediment exposure can affect the cycling of nutrients (e.g., nitrogen, phosphorus), the release and valence conversion of heavy metals, and the desorption and degradation of organic pollutants in the overlying water. This can further affect the community structure of aquatic organisms. The aim of this paper is to analyze the relevant literature on freshwater sediment dredging, and to summarize the current knowledge of the potential environmental risks caused by the dredging and utilization of freshwater sediments. Based on this, the paper attempts to propose suggestions to mitigate these adverse environmental impacts. These are significant contributions to the development of environmentally friendly freshwater sediment dredging technologies.

Sedimentary organic matter molecular composition reveals the eutrophication of the past 500 years in Lake Daihai, Inner Mongolia

Lake eutrophication seriously threatens water quality and human health. Under continuous global warming and intensified human activity, increasing attention is being paid to how lake trophic status responds to climate change and anthropogenic impacts. Based on the sedimentary organic matter (SOM) molecular composition determined by the Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) technology, and combined with the SOM stable nitrogen isotopes (δ¹⁵N_org), we studied how lake trophic status and ecology respond to both climatic changes and anthropogenic impacts of the past 500 yrs at Lake Daihai, Inner Mongolia. The results show that the relative abundance of lipids, proteins, and carbohydrates in lake sediments kept relatively low before AD ∼1850, and increased gradually thereafter, especially after AD ∼1950, suggesting that the lake trophic status was low before AD ∼1850, but obviously increased during the past one more century. On the other hand, the relative abundance of allochthonous condensed aromatics and vascular plant-derived polyphenols compounds gradually decreased after AD ∼1850, which is most likely due to the intensified land-use changes in the catchment. Our results show that the SOM molecular composition is more sensitive to trace the land-use changes than the δ¹⁵N_org ratios, suggesting a potential use of this technique to trace even earlier human land uses (e.g., during the prehistorical times) in a catchment. The results of this study suggest that intensified land-use change, increased discharges of human sewage and industrial wastewater, cropland runoff, and concentrated effects caused by lake level drops may have combinedly increased nutrient concentration and accelerated lake eutrophication at Lake Daihai. Therefore, proper policy is necessary to slow down anthropogenic impacts and limit further eutrophication for lakes like Lake Daihai.

Electrokinetic remediation leads to translocation of dissolved organic matter/nutrients and oxidation of aromatics and polysaccharides

Dissolved organic matter (DOM) in the sediment matrix affects contaminant remediation through consumption of oxidants and binding with contaminants. Yet the change in DOM during remediation processes, particularly during electrokinetic remediation (EKR), remains under-investigated. In this work, we elucidated the fate of sediment DOM in EKR using multiple spectroscopic tools under abiotic and biotic conditions. We found that EKR led to significant electromigration of the alkaline-extractable DOM (AEOM) toward the anode, followed by transformation of the aromatics and mineralization of the polysaccharides. The AEOM remaining in the cathode (largely polysaccharides) was resistant to reductive transformation. Limited difference was noted between abiotic and biotic conditions, indicating the dominance of electrochemical processes when relatively high voltages were applied (1-2 V/cm). The water-extractable organic matter (WEOM), in contrast, showed an increase at both electrodes, which was likely attributable to pH-driven dissociations of humic substances and amino acid-type constituents at the cathode and the anode, respectively. Nitrogen migrated with the AEOM toward the anode, but phosphorus remained immobilized. Understanding the redistribution and transformation of DOM could inform studies on contaminant degradation, carbon and nutrient availability, and sediment structural changes in EKR.

Shifts in the bacterial community caused by combined pollutant loads in the North Canal River, China

A typical anthropogenically disturbed urban river polluted by a combination of conventional pollutants (nitrogen and phosphorus pollution) and heavy metals was investigated along a 238 km stretch. Changes in the bacterial community were evaluated using high-throughput sequencing, and the relationships between bacteria, heavy metals, and conventional pollutants were investigated. There was large spatial heterogeneity in the bacterial community along the river, and bacterial diversity in the upstream and midstream sections was much higher than in the downstream section. Heavy metals and conventional pollutants both exhibited close correlations with bacterial diversity and composition. For instance, potential fecal indicator bacteria, sewage indicator bacteria and pathogenic bacteria, such as Ruminococcus and Pseudomonas, were closely associated with Cu, Zn, and NH₄⁺-N. Rather than conventional pollutants, heavy metals were the main driving factors of the microbial community characteristics. These results confirm that bacterial communities play a crucial role in biogeochemical cycles. Therefore, heavy metals could be used as biomarkers of complex pollution to indicate the pollution status of riverine ecosystems and contribute to the restoration of habitats in anthropogenically disturbed urban rivers.

Stimulation of high-concentration dissolved nitrogen and reactive phosphorus in Lake Taihu sediments on the initiation and maintenance of cyanobacterial blooms

The spatio-temporal variation of dissolved inorganic nitrogen (DIN: NH₄⁺-N, NO₂^--N, and NO₃^--N) and dissolved reactive phosphorus (DRP) in Meiliang Bay of Lake Taihu sediments and their potential effects on the cyanobacterial blooms were studied. Monthly sampling was performed using high-resolution dialysis sampling devices (HR-Peeper) and two important results were observed in April (the initiation period of cyanobacterial bloom) and June-August (the maintenance period of cyanobacterial blooms). In April, high concentrations of dissolved NO₂^--N and NO₃^--N, probably caused by the groundwater influx, were observed in deep anoxic sediments (below 110 mm). NO₂^--N and NO₃^--N are good electron acceptors for the mineralization of organic P under anaerobic conditions and should lead to an increase in DRP concentrations in sediments, DRP released from sediments thus further stimulating the cyanobacterial growth and the outbreak of severe cyanobacterial blooms in May due to the extremely low concentrations of DRP in the water body. From June to August, high concentrations of NO₂^--N, NO₃^--N, and DRP were observed in the surface sediment, which was caused by the release of NH₄⁺-N from the mineralization of cyanobacterial debris. This should play an important role in maintaining cyanobacterial growth, especially in stimulating the occurrence of cyanobacterial blooms during September, when N and P were co-limited. This study revealed high-concentration DIN and DRP in Lake Taihu sediments potentially stimulated the initiation and maintenance of cyanobacterial blooms.

Applying EEM-PARAFAC combined with moving-window 2DCOS and structural equation modeling to characterize binding properties of Cu (II) with DOM from different sources in an urbanized river

Dissolved organic matter (DOM) in aquatic environment distinctly affects the behavior and fate of heavy metals via complexation, while the interfacial mechanisms and processes are still lacking in detail. Here, Cu (II) binding characteristics of DOM originated from hilly (NDOM), rural (RDOM) and urban (UDOM) regions in an urbanized river was explored by fluorescence excitation-emission matrix spectroscopy (EEM) combined with principal component coefficients, parallel factor analyses (PARAFAC), moving-window two-dimensional correlation spectroscopy (MW2DCOS) and structural equation modeling (SEM). Eight components were extracted from the titrants through EEM-PARAFAC, i.e., phenol-like substance (C1), tyrosine-like substance (C2), visible tryptophan-like substance (C3), ultraviolet tryptophan-like substance (C4), recent biological production (C5), wastewater-derived organic matter (C6), microbial humic-like substance (C7) and fulvic-like substance (C8). Interestingly, NDOM only contained C1, C3, C5 and C8, while nearly all components were found in RDOM (except for C2) and UDOM (except for C4). The f value of C1 (1.239) in NDOM was much higher than those in RDOM (0.134) and UDOM (0.115), so was of C8. It indicated that phenol-like and fulvic-like derived from autochthonous sources exhibited great binding ratios in the complexation with Cu (II). Moreover, C3 and C5 from UDOM exhibited higher f values (0.591 and 1.983) than those from NDOM and RDOM, suggesting that Cu (II) has a great binding capacity on protein-like from domestic and industrial wastewater. The MW2DCOS revealed that phenol-like and protein-like in NDOM and RDOM were essential for the binding of 160 μmol L^-1 Cu (II), whereas fulvic-like in NDOM and UDOM could react significantly with 10 μmol L^-1 Cu (II). Based on SEM, Cu (II) concentration had a negative direct effect on the fluorescence intensity of C7 or C8, whereas it showed an indirect positive effect on C7 or C8 through influencing C5, so was C6. It suggested that Cu (II) showed an indirect positive effect on the C8. This study might present a further comprehend of the environmental behaviors of Cu (II) in rivers.

Reconsidering operation pattern for cation-exchange resin assistant anaerobic fermentation of waste activated sludge: Shorting residence period towards dosage-reduction and anti-fouling

Short-chain fatty acids (SCFAs) generation through anaerobic fermentation has been regarded as a promising pathway to achieve carbon recovery and economic benefits in waste activated sludge management. Despite the cation exchange resin (CER) assistant anaerobic fermentation strategy has been previously reported for enhancing anaerobic fermentation, the overlarge CER usage and serious CER pollution have limited its engineering application. This study provided a reconsideration for the operation pattern modification. Through 4-day anaerobic fermentation with CER residence period shrinking to 1 day, 40.9% sludge VSS solubilization and reduction were achieved, triggering a considerable sludge hydrolysis rate of 28.4%. Thereby, SCFAs production was improved to 264.8 mg COD/g VSS. Such performances were approximately 80.2-87.8% of those with conventional CER residence period (8 days). The organic composition distribution and parallel factor analysis demonstrated that similar biodegradability and utilizability of fermentative liquid were achievable with various operation patterns. Compared with the conventional operation pattern, the modified operation pattern with shortened CER residence period (1 day) also displayed satisfying anaerobic fermentation efficiency and numerous engineering bene fits, e.g. decreased CER usage, reduced engineering footprint, relieved CER fouling, and increased operation convenience. The findings might provide sustainable development for CER assistant anaerobic fermentation strategy and enlighten the direction of anaerobic fermentation process.

Characterization of Dissolved Organic Matter in Solar Ponds by Elemental Analysis, Infrared Spectroscopy, Nuclear Magnetic Resonance and Pyrolysis-GC-MS

The abundance and chemical composition of dissolved organic matter (DOM) in the brine of solar ponds affect the efficiency of mineral extraction and evaporation rates of the brine, and cause undesired odor and color of the products. Here, we report an investigation into the composition and changes of DOM in solar ponds from Salt Lake brine with multiple complementary analysis techniques. The results showed that the DOM derived from Salt Lake brine was primarily composed of carbohydrates, aliphatic and aromatic compounds. The concentrations of dissolved organic carbon in solar ponds increased with exposure time by up to 15−fold (from 23.4 to 330.8 mg/L) upon evaporation/irradiation of Salt Lake brine. Further qualitative analyses suggest that the relative abundance of aliphatic compounds (including functionalized ones) increased from 49.5% to 59.2% in the solar pond process, while the opposite was observed for carboxylic acid moieties, aromatics and carbohydrates, which decreased from 15.7%, 7.1% and 26.1% to 13.4%, 5.3% and 23.0%, respectively. The pyrolysis–gas chromatography–mass spectrometry results reveal that the presence of some sulfur-containing organics implied some anaerobic biotic decay, but microbiological processes were probably subordinate to photo-induced DOM transformations. In the Salt Lake brine, exposure-driven decay decreased the abundance of polysaccharides and increased that of mono- and polyaromatic pyrolysis products. Our results here provide new insights for better understanding the changes of DOM chemical composition in the solar ponds of Salt Lake brine.

How do drying-wetting cycles influence availability of heavy metals in sediment? A perspective from DOM molecular composition

Investigating the influence mechanism of drying-wetting cycles on the availability and mobility of heavy metals in sediment from the perspective of the molecular composition of dissolved organic matter (DOM) may gain a new understanding, but little current information exists. Here, we used spectral technologies, high-resolution mass spectrometry, and elemental stoichiometry method to trace the change rules of the molecular composition of DOM in the riparian sediment of the river. Results showed that the drying-wetting cycles could benefit the degradation of labile fractions (e.g., proteins, aliphatics, and lipids) of DOM and retain the fractions with high aromaticity and molecular size (e.g., lignin). The decrease in the availability of Cd after drying-wetting alternation processes was highly related to these changes in DOM composition. However, the availability of Zn and Cu remained almost unchanged, which probably resulted from the release and depletion of N and S in sediment-derived DOM under drying-wetting alternation conditions. As for Cr, its exchangeable fraction was unchanged during the drying-wetting alternation process, likely due to its high stability in the sediment. These results have implications on the environmental geochemical cycling of heavy metals in the riparian sediment with frequent drying-wetting alternation.

Effects of pyrolysis temperature on proton and cadmium binding properties onto biochar-derived dissolved organic matter: Roles of fluorophore and chromophore

Understanding the environmental behavior of biochar-derived dissolved organic matter (BDOM) is crucial for promoting the extensive utilization of biochar and meeting the carbon neutrality targets. However, limited studies focused on the binding mechanism of protons and Cd with DOM released from biochar produced at different pyrolysis temperatures. By combining excitation-emission matrix spectroscopy and parallel factor analysis, we found that the humic-like fluorophores in BDOM had higher aromaticity, molecular weight, and contents of carboxylic and phenolic groups relative to the protein-like fluorophores. Conversely, the protein-like fluorophores exhibited a stronger binding affinity for Cd than humic-like fluorophores. With the pyrolysis temperature increased from 300 °C to 500 °C, the quenching effects of Cd on the protein-like components were enhanced significantly. Their fluorescence intensities could be quenched up to 51.64%. The results of ultraviolet-visible absorbance spectroscopy and differential absorbance spectroscopy showed that the carboxylic-like and phenolic-like chromophores were involved in the protons and Cd binding process of BDOM. The binding ability of phenolic-like chromophores with Cd was reduced as a function of increasing pyrolysis temperature. These findings implied that these carboxylic and phenolic groups were mainly contained in the non-fluorescent components. Besides, protons and Cd could also induce inter-chromophore interactions in BDOM, and the interaction was proportional to the pyrolysis temperature. These results clearly demonstrated the pyrolysis temperature-dependent changes in the protons and Cd binding properties of BDOM. More importantly, the possible risk of Cd mobility caused by the protein-like components in BDOM cannot be ignored when the biochar was applied in contaminated soils. This research extends our knowledge of the application potentiality of biochar in heavy metal polluted soil.

The molecular characteristics of dissolved organic matter in urbanized river sediments and their environmental impact under the action of microorganisms

The complex relationships between the molecular composition of dissolved organic matter (DOM) and microbial communities are essential for maintaining the stability of aquatic ecosystems. This study comprehensively analyzed the characteristics and potential effects of DOM molecular composition as well as the relationship between microbial communities and DOM molecular composition in sediments from the Beiyun River, Beijing, China. The results showed that the content of DOM in Beiyun River sediments was 9.93-41.57 g/kg, mainly composed of lignin-like (36.75%) and protein-like (17.79%) substances. Compared with other rivers affected by anthropogenic activities, the higher content of labile substances in the Beiyun River increased the risk of nutrient release. At the same time, 1402 molecules remained stable in each sample, most of which were refractory lignin-like substances and protein-like substances carrying ester groups. The agricultural section contained more common DOM molecules than the urban section, mainly tannin-like and lignin-like substances with unsaturated or cyclic structures. And, the intensity of anthropogenic activities was the main reason affecting the diversity of unique DOM molecular in each sample. Moreover, Dechloromonas as the dominant genus of Proteobacteria was closely related to the biological modification of low unsaturated (DBE < 15) condensed aromatic compounds (P < 0.05). Anaerolineaceae and Anaerolineae belonging to the Chloroflexi phylum have the potential to degrade medium and high molecular weight (M/Z > 400) liable substances (P < 0.05) and release lignin-like substances. In addition, the proportion of protein-like substances can indirectly reflect the risk of nutrient release in sediments affected by urbanization. Thus, the results of this study further reveal the impact of urbanization on rivers, and provide theoretical basis and guidance for pollution control of the Beiyun River and other urbanized rivers worldwide.

Properties of sediment dissolved organic matter respond to eutrophication and interact with bacterial communities in a plateau lake

Sediment dissolved organic matter (DOM) in inland waters is commonly affected by environmental changes. However, knowledge about how sediment DOM responds to eutrophication and the associations between sediment DOM and bacterial communities requires further investigation. We selected a sediment core from Dianchi Lake (China) that was dated from 1864 to 2019 by the activity of radionuclides (²¹⁰Pb and ¹³⁷Cs). δ¹³C_DOC changes fit well with the historical record that heavy eutrophic status in Dianchi Lake were observed since 1980s. Large amounts of dissolved organic carbon (DOC), chromophoric (CDOM) and fluorescent (FDOM) DOM accumulated at the top of the sediments during the eutrophication period (1982-present). The additional algae sources with a higher degradation rate altered the composition, aromaticity and humification of DOM. After long-term mineralization, the remaining DOM became more and more recalcitrant and kept a relatively stable level at older sediments. A co-occurrence network analysis revealed that Proteobacteria, Chloroflexi, Acidobacteriota, Bacteroidota and Desulfobacterota were the most abundant species at the phylum level and clustered into three primary modules. Different microbes shared unique preferences for niches, causing a heterogeneous bacterial distribution at different depths. We conducted Spearman's correlation and redundancy analysis (RDA) to explore potential interactions between bacterial community and sediment DOM. The richness and diversity of bacterial communities were positively related to DOM content, suggesting abundant DOM can produce more available resources for bacteria. RDA results showed some specific species might modify DOM composition and structure. This study suggests that sediment DOM properties were regulated by source transformation during eutrophication, and emphasizes the importance of microbial role on sediment biogeochemical process.

Elevated temperature altered the binding sequence of Cd with DOM in arable soils

Dissolved organic matter (DOM) is one of the most active soil components, which plays pivotal roles in the migration and fate of heavy metals in soils. The interactions of heavy metals with DOM are controlled by the structure and properties of DOM. The changes of temperature have a significant effect on the content and composition of DOM and thus may affect the binding nature of heavy metals with DOM. In the current study, we conducted a 180-d incubation experiment with an arable soil at temperatures of 15, 30 and 45 °C. Fluorescence spectroscopy was used to examine the composition of DOM and two-dimensional correlation spectroscopy was applied to determine the binding intensity and sequence between cadmium (Cd) with DOM. Two humic-like substances (C1, C3) and a protein-like substance (C2) were identified from soil DOM. Elevated temperature changed the characteristic and structure of DOM. The humification degree and aromaticity of DOM increased from 15 °C to 30 °C but decreased at high temperature (45 °C). The alterations in temperature exert no impact on the type of organic functional groups in DOM binding with Cd. However, elevated temperature changed the binding sequence of Cd with DOM fractions. Polysaccharide, phenolic, and aromatic groups exhibited the fastest response to Cd at 15, 30, and 45 °C, respectively. These observations would provide a better understanding on the environmental behavior of Cd in arable soils under the context of global warming.

Insight into interactions of heavy metals with livestock manure compost-derived dissolved organic matter using EEM-PARAFAC and 2D-FTIR-COS analyses

Dissolved organic matter (DOM), as the most active ingredient in compost, directly determines the speciation and environmental behavior of HMs. Here, the binding properties of DOM derived from chicken-manure compost (CHM), cow-manure compost (COM) and pig-manure compost (PIM) with HMs were explored by analyses of Fluorescence excitation-emission matrix parallel factor (EEM-PARAFAC) and two-dimensional correlation Fourier transform infrared spectroscopy (2D-FTIR-COS). Results showed that the binding characteristics vary with origin of DOM and type of HMs. The fulvic-like component dominated the transformation of HMs speciation, and CHM-DOM had higher affinity with HMs and greater risk causing pollution due to its higher aromaticity, molecular weight and distribution of fluorescent components. Moreover, Cu(II) can efficiently bind to DOM with the stability constants (log k_M) ranging from 4.53 to 5.38, followed by Pb(II) (3.34-3.57), whereas Cd(II) can hardly bind to DOM. The amide and polysaccharide were the predominant sites for HMs binding in CHM-DOM, and polysaccharide and phenolic in COM-DOM, while phenolic and amide in PIM-DOM, respectively. Although the proportion of protein-like components and non-fluorescent polysaccharides in DOM were low, their role in HMs binding should not be ignored. In brief, the environmental risk caused by livestock manure compost may originate from interactions between DOM and HMs.

Mechanisms of photochemical release of dissolved organic matter and iron from resuspended sediments

Photochemical reactions can alter the transformation of sedimentary organic matter into dissolved organic matter (DOM) and affect its ultimate fate in water ecosystems. In the present study, the photorelease of DOM and Fe from resuspended lake sediments was investigated under different O₂ and NO₃^- concentration conditions to study the mechanisms of DOM and Fe photorelease. The amount of photoreleased Fe, which ranged from 0.22 to 0.70 μmol/L, was significantly linearly correlated with the amount of photoreleased DOM. O₂ and NO₃^- could promote the photochemical release of DOM and Fe, especially during the initial 4 h irradiation. In general, the order of the photorelease rates of DOM and Fe under different conditions was as follows: NO₃^-/aerobic > aerobic ≈ NO₃^-/anaerobic > anaerobic. The photorelease rates of DOM and Fe were higher for the initial 4 hr irradiation than these for the subsequent 8 hr irradiation. The photorelease of DOM and Fe is thought to proceed via direct photodissolution and indirect processes. The relative contributions of indirect processes (>60%) was much greater than that of direct photodissolution (<40%). The photoproduced H₂O₂ under aerobic and anaerobic conditions indicated that hydroxyl radicals (•OH) are involved in the photorelease of DOM. Using •OH scavengers, it was found that 38.7%, 53.7%, and 77.6% of photoreleased DOM was attributed to •OH under anaerobic, aerobic, and NO₃^-/aerobic conditions, respectively. Our findings provide insights for understanding the mechanisms and the important role of •OH in the DOM and Fe photorelease from resuspended sediments.

Phosphate hinders the complexation of dissolved organic matter with copper in lake waters

The properties of phosphate in lakes and their ability to cause eutrophication have been well studied; however, the effects of phosphate on the environmental behavior of other substances in lakes have been ignored. Dissolved organic matter (DOM) and heavy metals may coexist with phosphate in lakes. Herein, the mechanisms underlying the influence of phosphate on heavy metals complexation with DOM were investigated using multi-spectroscopic tools. Overall, the amount of DOM-bound Cu(Ⅱ) decreased with the increasing phosphate content. Furthermore, the fluorescence excitation and emission matrix results combined with parallel factor analysis showed that when the Cu(Ⅱ) concentration increased from 0 to 5 mg/L and 50 μM phosphate to the reaction of DOM and copper, the fluorescence intensity of tyrosine (component 1), humic-like (component 2) and tryptophan (component 3) decreased by 36.46%, 57.34%, and 74.70% compared with the treatment with no phosphate addition, respectively. This finding indicates that the binding of different fluorescent components to Cu(Ⅱ) was restricted by phosphate. Furthermore, different functional groups responded differently to Cu(Ⅱ) under different phosphate concentrations. The binding sequence of different functional groups under high concentration of phosphate (phenolic hydroxyl group＞amide (Ⅰ) ＞carbohydrates) was completely opposite to that with no phosphate. These results demonstrated that phosphate could restrict the binding affinity of heavy metals with different fluorescent substances or organic ligands of DOM, suggesting that the comigration of DOM-bound heavy metals in lakes is hindered by phosphate and the risk of heavy metal poisoning in aquatic organisms is therefore diminished.

Advances in the characterization and monitoring of natural organic matter using spectroscopic approaches

Natural organic matter (NOM) is ubiquitous in environment and plays a fundamental role in the geochemical cycling of elements. It is involved in a wide range of environmental processes and can significantly affect the environmental fates of exogenous contaminants. Understanding the properties and environmental behaviors of NOM is critical to advance water treatment technologies and environmental remediation strategies. NOM is composed of characteristic light-absorbing/emitting functional groups, which are the "identification card" of NOM and susceptive to ambient physiochemical changes. These groups and their variations can be captured through optical sensing. Therefore, spectroscopic techniques are elegant tools to track the sources, features, and environmental behaviors of NOM. In this work, the most recent advances in molecular spectroscopic techniques, including UV-Vis, fluorescence, infrared, and Raman spectroscopy, for the characterization, measurement, and monitoring of NOM are reviewed, and the state-of-the-art innovations are highlighted. Furthermore, the limitations of current spectroscopic approaches for the exploration of NOM-related environmental processesand how these weaknesses/drawbacks can be addressed are explored. Finally, suggestions and directions are proposed to advance the development of spectroscopic methods in analyzing and elucidating the properties and behaviors of NOM in natural and engineered environments.

Application of Fourier transform ion cyclotron resonance mass spectrometry to characterize natural organic matter

Advances in the ultra-high-resolution mass spectroscopy lead to a deep insight into the molecular characterization of natural organic matter (NOM). Fourier transform ion cyclotron resonance mass spectrometry (FTICR-MS) has been used as one of the most powerful tools to decipher NOM molecules. In FTICR-MS analysis, the matrix effects caused by the co-occurring inorganic substances in water samples greatly affect the ionization of NOM molecules. The inherent complexity of NOM may hinder its component classification and formula assignment. In this study, basic principles and recent advances for sample separation and purification approaches, ionization methods, and the evolutions in formula assignment and data exploitation of the FTICR-MS analysis were reviewed. The complementary characterization methods for FTICR-MS were also reviewed. By coupling with other developed/developing characterization methods, the statistical confidence for inferring the NOM compositions by FTICR-MS was greatly improved. Despite that the refined separation procedures and advanced data processing methods for NOM molecules have been exploited, the big challenge for interpreting NOM molecules is to give the basic structures of them. Online share of the FTICR-MS data, further optimizing the FTICR-MS technique, and coupling this technique with more characterization methods would be beneficial to improving the understanding of the composition and property of NOM.

Dissolved organic matter released from rice straw and straw biochar: Contrasting molecular composition and lead binding behaviors

It remains debatable whether carbonized straw reapplying is a better solution than direct straw reapplying. Comparison of the characteristics and complexation behaviors of dissolved organic matter (DOM) derived from straw (ST) and biochar (BC) may offer new insights, but little current information exists. Herein, DOM samples were characterized by Fourier transform ion cyclotron resonance mass spectrometry (FTICR-MS), revealing that the molecular weight and condensed aromatic components of BCDOM (457.70 Da and 71.16%, respectively) were higher than those of STDOM (433.48 Da and 3.13%, respectively). In particular, the N-containing compounds of BCDOM was more aromatic than STDOM. By combining spectroscopic techniques, complexation modeling, and chemometric analysis, BCDOM was shown to exhibit higher binding parameters (log K_M) and more binding sites for Pb than STDOM. Noteworthily, the two binding sites, aromatic NO and aromatic NO₂, existed only in the interaction of BCDOM with Pb. Furthermore, while phenol-OH displayed the fastest response to Pb in both STDOM and BCDOM, the binding sequences were not exactly the same. These differences may be related to the variations in the aromaticity and N-containing structures of DOM detected by FTICR-MS. These findings have implications on the stewardship of straw- and biochar-amended soil.

Spectroscopic response of soil organic matter in mining area to Pb/Cd heavy metal interaction: A mirror of coherent structural variation

Understanding the interaction between heavy metals and soil organic matter (SOM) in mining area is important for the clarification of the environmental behaviors of heavy metals. In this work, the coherence of structural changes of SOM during interaction with Pb²⁺ and Cd²⁺ ions were examined by using UV-vis/fluorescence spectroscopy coupled with correlation analyses. The result showed that phenolic- and carboxylic-like groups of SOM were engaged in the complexation of heavy metals (Pb²⁺ and Cd²⁺) with SOM, resulting in the formation of highly conjugated macromolecules/aggregates and an increase in molecular weight/size. Fluorescent humic-like, fulvic-like, and protein-like species were involved in the binding with Pb²⁺/Cd²⁺ ions, which were closely correlated with phenolic-like and carboxylic-like constitutes. SOM was more favorable to bind with Pb²⁺ ions than Cd²⁺ ions, with a less susceptive of SOM structure to Pb²⁺/Cd²⁺ ions in the mining area compared to those off the mining area under heavy metal stress. These results may provide a new insight for the treatment and remediation of heavy metal-polluted soil in mining area.

The composition difference of macrophyte litter-derived dissolved organic matter by photodegradation and biodegradation: Role of reactive oxygen species on refractory component

The overgrowth of macrophytes has become serious due to increasing eutrophication in shallow lakes. The primary degradation processes of macrophytes litter, including photodegradation and biodegradation, induce considerable patchiness in the concentration and bioavailability of dissolved organic matter (DOM). In this study we investigated the composition evolution of DOM derived from emergent aquatic plant litter, Phragmites australis, in microbial degradation, photodegradation, and the combination of bio- and photo-degradation. Results revealed that the effects of photo- and biodegradation on the composition difference of macrophyte litter-derived DOM during short- and long-term degradation phase were different. Although large changes in DOM were observed after five days of incubation, the abundance and chemical composition were similar in the three treatments. However, more concentration of DOM was produced by combined photo- and biodegradation at the long-term degradation phase, and the composition of DOM showed less lignin-like formulae, as well as less condensed aromatic and aromatic compounds when compared to sole treatments. More reactive oxygen species (ROS) were found under the combined treatments, thus the contents of refractory components (condensed aromatic- and aromatic compound groups) were reduced. This study provide deeper insight into the fate of DOM and relevant biogeochemical processes in eutrophic lakes.

Impact of drying/wetting conditions on the binding characteristics of Cu(ii) and Cd(ii) with sediment dissolved organic matter

The biogeochemical processing of dissolved organic matter (DOM) in bottomland sediment under drying/wetting conditions regulates the environmental behavior of heavy metals. Although moisture is a critical factor, the structural characteristics of DOM and its reactivity with heavy metals under drying/wetting conditions are not well known. Herein, the response of DOM to drying/wetting conditions and its influence on the binding of Cu(ii) and Cd(ii) onto DOM were clarified via various multi-spectroscopic techniques. Ultraviolet-visible spectra (UV-Vis) showed that higher aromatic, hydrophobic, and molecular weight fractions were observed in sediment DOM under drying conditions than those under wetting conditions. The binding abilities for Cd(ii) with DOM under drying/wetting conditions are lower than those for Cu(ii). The stability constants between Cu(ii) and DOM were found to decrease under drying/wetting conditions; however, the binding capacities for Cu(ii) increased, especially under wetting conditions. Two-dimensional correlation spectroscopy based on Fourier-transform infrared (FTIR) and synchronous fluorescence spectra (SFS) showed that Cu(ii) and Cd(ii) have different binding sequences and binding sites and that Cu(ii) has more binding sites under drying and wetting conditions; however, Cd(ii) shows the opposite behavior. These results clearly demonstrate that the binding of sediment DOM with Cu(ii) is more prevalent and stable compared with Cd(ii) under drying and wetting conditions. Because of its relatively low binding capacity and binding stability, Cd(ii) can exhibit a high environmental hazard for migration and transformation with DOM due to water flow under wetting conditions. This study helps reveal the impact of drying/wetting conditions on the environmental behavior of heavy metals in bottomland wetlands.

The biogeochemical processing of dissolved organic matter (DOM) in bottomland sediment under drying/wetting conditions regulates the environmental behavior of heavy metals.

Level and distribution of nutrients in the hyporheic zone of Lake Taihu (China) and potential drivers

To better understand the nutrient distributions in the hyporheic zone of a large shallow eutrophic lake (Lake Taihu) and the potential drivers, the total organic carbon (TOC), total nitrogen (TN), and total phosphorus (TP) were investigated in March and December 2016. Spatial differences in the TN and TP contents existed in the hyporheic zone, particularly between the eastern and southern zones and the western and northern zones. The TOC/TN ratios in the western, northern, and central zones were mostly <8 and even reached 4, indicating that organic matter originated from aquatic organisms and algae, whereas those in the southern and eastern zones exhibited wide ranges, indicating complex pollution sources. The chloride depth profiles suggested that upwelling hyporheic flow potentially occurred in the southern, western, and northern zones, while alternating flow directions occurred in the eastern zone and no flow or weak flow occurred in the central zone. Compared to the 1st investigation, the TOC, TN, and TP in sediments in the 2nd investigation increased by 13%, 41%, and 87%, respectively, and these changes were mostly due to large hyporheic fluxes with high nutrient concentrations from shallow groundwater. The behavior of the hyporheic zone as an active pollution source/sink due to hyporheic flow should be considered in the comprehensive management of Lake Taihu. PRACTITIONER POINTS: Depth profiles of nutrients differed between sampling sites and between zones of Lake Taihu due to different pollution sources. Nutrients in sediment increased largely during winter compared to spring due to potential groundwater pollution through upwelling flow. No significant difference in sediment total organic carbon and ratios of C/N indicates a similar internal pollution source lake wide.

Spectroscopic study of the effects of dissolved organic matter compositional changes on availability of cadmium in paddy soil under different water management practices

It is well established that water management can influence the availability of Cd in paddy soil but the role of dissolved organic matter (DOM) characteristics in this process is still unclear. Here, we measured and compared the DOM quantity and quality between flooded and wetted treatments by spectroscopic and chemometric analysis and applied correlation analysis to relate DOM characteristics with availability concentrations of Cd. Ultraviolet-visible showed that aromaticity and hydrophobicity of DOM significantly decreased with time in wetted paddy soil (p < 0.05) but had no significant difference in flooded paddy soil (p > 0.05). According the results from two-dimensional correlation spectroscopy analytical method, humic- and protein-like substances had fast response during cultivation process. Two humic-like substances (C1, C2) and two protein-like substances (C3, C4) were identified from paddy soil-derived DOM by combining emission and excitation matrix spectroscopy with parallel factor. Compared to component C1, C3, and C4, component C2 has stronger aromaticity and hydrophobicity and higher molecular size (665-1000 Da). Its proportion declined markedly during the wetting periods but increased slightly during flooding. Pearson correlation analysis illustrated that flooding was more helpful in immobilizing Cd than wetting due to the aromatic, hydrophobic, and high molecular weight constituents remained in flooded treatments and the substantial decomposition of component C2 in wetted treatments. These results suggested that spectroscopic and chemometric methods are helping to further explain the impacts of DOM quality on Cd availability under different water management practices.

Molecular weight-dependent spectral and metal binding properties of sediment dissolved organic matter from different origins

The metal binding potential of dissolved organic matter (DOM) is highly related with its inherent properties such as molecular weight (MW). Here sediment DOMs with different origins, i.e., algae- and macrophyte-dominated sediment DOM (named as ASDOM and MSDOM, respectively), were size-fractionated into low MW (LMW-, <1 kDa) and high MW (HMW-, 1 kDa~0.45 μm) fractions, with the spectral and metal binding properties in different MW fractions exploring via total organic carbon, absorption spectroscopy, fluorescence parallel factor (PARAFAC) analysis, and Cu(II) titration techniques. The MSDOM contained more organic carbon, lower specific UV absorbance, lower fluorescence index, higher humification index, and lower biological index compared to the ASDOM. As for the MW-fractionated samples, the humic- and fulvic-like fluorophores were mainly distributed in the LMW-DOM, while the protein-like ones were located richly in the HMW-DOM. Thus, obvious MW- and origin-dependent heterogeneities in abundance and spectral properties were observed for sediment DOMs. One humic-like, one fulvic-like, and two protein-like fluorescent components were identified by PARAFAC analysis, with different components exhibiting different variable patterns in response to Cu(II) addition. Irrespective of sample sources, humic- and fulvic-like components had higher condition stability constants (logK_M > 4.96) than protein-like components (logK_M < 4.86). As for the MW-fractionated samples, the HMW-DOM, especially the HMW humic-/fulvic-like components, exhibited higher metal binding potential than the bulk and LMW counterparts. This study highlighted the importance of HMW-DOM in manipulating the behavior, fate, and bioavailability of heavy metals in lake sediments.

Characterizing Properties and Environmental Behaviors of Dissolved Organic Matter Using Two-Dimensional Correlation Spectroscopic Analysis

Dissolved organic matter (DOM) exists ubiquitously in environments and plays critical roles in pollutant mitigation, transformation, and organic geochemical cycling. Understanding its properties and environmental behaviors is critically important to develop water treatment processes and environmental remediation strategies. Generalized two-dimensional correlation spectroscopy (2DCOS), which has numerous advantages, including enhancing spectral resolution and discerning specific order of structural change under an external perturbation, could be used as a powerful tool to interpret a wide range of spectroscopic signatures relating to DOM. A suite of spectroscopic signatures, such as UV-vis, fluorescence, infrared, and Raman spectra that can be analyzed by 2DCOS, is able to provide additional structural information hiding behind the conventional one-dimensional spectra. In this article, the most recent advances in 2DCOS applications for analyzing DOM-related environmental processes are reviewed, and the state-of-the-art novel spectroscopic techniques in 2DCOS are highlighted. Furthermore, the main limitations and requirements of current approaches for exploring DOM-related environmental processes and how these limitations and drawbacks can be addressed are explored. Finally, suggestions and new approaches are proposed to significantly advance the development of 2DCOS in analyzing the properties and behaviors of DOM in natural and engineered environments.

Seasonal changes of lead mobility in sediments in algae- and macrophyte-dominated zones of the lake

This study examined lead (Pb) pollution in algae- and macrophytes-dominated sediments, using diffusive gradient in thin films (DGT) and dialysis (HR-Peeper) techniques. Lead pollution varied by season in the two different ecotype sediments. In the algae-dominated zone, the highest concentrations of DGT-labile Pb and dissolved Pb occurred in April and July, respectively. The reductive dissolution of Fe/Mn oxides was identified as an important driver for Pb releases in April and July. This was supported by the decrease of the reducible fraction of Pb in sediments during those sampling periods. Furthermore, dissolved organic matter (DOM) complexation with Pb in sediments also significantly increased the dissolved Pb concentrations in July. The Pb-DOM complexes accounted for 95% of the total chemical species of Pb in pore water, calculated by Visual MINTEQ 3.1 model. Low concentrations of labile and dissolved Pb were observed in October and January; these resulted from the formation of Pb-sulfide precipitates and adsorption by Fe/Mn oxides. It was supported by the high rate of Pb(HS)₂ precipitation (saturation index > 0), at 36%, in October samples and the high reducible fraction of Pb in sediments in January samples. In the macrophytes-dominated region, there was a decrease of labile and dissolved Pb concentrations in April and July. It is likely because of the uptake of Pb by submerged macrophyte roots and the Fe/Mn plaques in the root surface. High concentrations of labile and dissolved Pb were observed in October and January, likely resulting from the DOM complexation with Pb in sediments. This was supported by the fact that the Pb-DOM complexes accounted for 90% and 87% of the total chemical species of Pb in October and January, respectively.

Investigating spectroscopic and copper-binding characteristics of organic matter derived from sediments and suspended particles using EEM-PARAFAC combined with two-dimensional fluorescence/FTIR correlation analyses

Different environmental dynamics of sediment organic matter (SOM) and suspended particulate organic matter (SPOM) result in great disparities in characteristics and subsequent interactions with heavy metals. In this study, sediments and suspended particles were collected from two large and shallow lakes (Taihu and Hongze Lake) to study the difference in the characteristics and copper binding properties between SOM and SPOM through spectroscopy method. Our results showed that SPOM in Taihu Lake was dominated by autochthonous tyrosine-like substance and SOM was dominated by terrestrial humic-like substance, whereas SPOM in Hongze Lake was dominated by terrestrial humic-like substances. Furthermore, dissolved organic matter (DOM) in Taihu and Hongze Lake was controlled by autochthonous protein and terrestrial humic substances, respectively. Comparison of SPOM between these two lakes indicated that various organic matter sources could be responsible for the organic matter characteristics in suspended particles. Meanwhile, relatively higher binding affinities and more binding sites were observed for SPOM in both two lakes compared to SOM through two-dimensional correlation spectroscopy (2D-COS) analysis of synchronous fluorescence spectra. Moreover, 2D-COS analysis of FTIR spectra revealed that hydrophobic groups (i.e., phenolic groups) had higher binding affinity than hydrophilic groups (i.e., polysaccharide groups) for both SOM and SPOM. Our results provide a new angle for understanding the suspended particles in shallow lakes, which might play a more important role in the environmental behaviors of heavy metals, than has been previously thought.

Application potential of biochar in environment: Insight from degradation of biochar-derived DOM and complexation of DOM with heavy metals

Biochar-derived dissolved organic matter (DOM) is important for determining the application potential of biochar in soil remediation. However, little is known about the degradation behavior of biochar-derived DOM and its interaction with heavy metals. Here, incubation experiments combined with quenching titration experiments, which analyzed by spectroscopic technology and chemometric method, were conducted to reveal such behaviors and mechanisms. Ultraviolet-visible (UV-Vis) spectra showed that high aromatic and hydrophobic fractions were enriched in biochar-derived DOM and enhanced during the cultivation process, thus the biochar-derived DOM may retain a high aromaticity, stability, and resistance. However, the environmental risk of Cu caused by the increase of DOM hydrophobicity cannot be overlooked while applying biochar to polluted soil. One fulvic-like (C1), one protein-like (C2) and two humic-like (C3, C4) substances were identified from biochar-derived DOM by using parallel factor analysis of excitation-emission matrix. Additionally, the fluorescence intensity variations of these components in DOM offered an additional interpretation for the observations from UV-Vis spectra. Two-dimensional correlation spectroscopy revealed that Cd binding to biochar-derived DOM first occurred in the protein- and fulvic-like fraction while protein- and humic-like substances had a stronger affinity for Cu. Furthermore, both phenolic and carboxyl groups firstly participated in the binding process of Cd with biochar-derived DOM, while polysaccharide gave the fastest response to Cu binding. These results clearly demonstrated the differences in specific heavy metal binding features of individual fluorescent substances and functional groups in biochar-derived DOM and contribute to improving the application effect of biochar in a multi-heavy metal polluted soil system.

Investigating organic matter properties affecting the binding behavior of heavy metals in the rhizosphere of wetlands

Soil organic matter (SOM) is a crucial factor affecting the immobilization of heavy metal in wetlands. Recent studies have shown that the rhizosphere SOM has great ability to immobilize heavy metals. However, there existed few works on studying molecular characteristics of SOM to explore the mechanisms. Electrospray ionization-Fourier transform ion cyclotron resonance-mass spectrometry (ESI-FTICR-MS) combined with FTIR spectroscopy were applied to investigate the characteristics of SOM in rhizosphere and nonrhizosphere samples and to find out what characteristics the rhizosphere SOM embodies conducive to metal binding in this paper. The rhizosphere contained higher C, P, Mn, and other metal concentrations. The adsorption of Cr on rhizosphere SOM was greater than that on nonrhizosphere SOM. Compared to nonrhizosphere SOM, rhizosphere SOM contained less saturated and more oxidized compounds, greater overall molecular weights (MW), more condensed aromatic structures (56.59% VS 51.56% by peak intensity), less carboxylate and N-containing COO functional groups (25.98% VS 56.63% by peak intensity), more hydrophilicity, and the latter four are conducive to metal binding. This study showed that the rhizosphere SOM had unique compositional and structural characteristics. These results provided evidence for the phytoremediation technologies of heavy metal contaminated wetlands.

Response of dissolved organic matter optical properties to net inflow runoff in a large fluvial plain lake and the connecting channels

Fluvial plain lake watersheds are usually highly urbanized and have high concentrations of chromophoric dissolved organic matter (CDOM). CDOM derived from the connecting urban channels usually share strong terrestrial and anthropogenic signals and net inflow runoff (Q_net) to the lake serves as a proxy of residential household sewage input. We investigate how Q_net controls the optical characteristics of CDOM in fluvial plain Lake Taihu and the connecting channels. CDOM absorption coefficient a(350), dissolved organic carbon (DOC), the fluorescence intensity (F_max) of seven PARAFAC components C1-C7, and δ¹⁵N-TDN were higher in the northwestern relative to the other lake regions, and a(250)/a(365), spectral slope S_275-295, and δ¹³C-DOM relative low in the northwestern lake, all indicating strong terrestrial and anthropogenic effects. Conversely, the urban land cover (%Cities), gross domestic product (GDP), and population density were relatively low in the western sub-watersheds and high in the eastern sub-watersheds. This apparent paradox reflects variations in Q_net from different sub-watersheds. Thus, significant positive relationships were found between Q_net and a(350), DOC, chemical oxygen demand (COD), chlorophyll-a (Chl-a), F_max of C1-C3 and C6-C7, and %C2-%C3 in the five hydraulic sub-watersheds. We revealed significant positive relationships between mean a(350), DOC, COD, Chl-a, C1-C3 and C6, %C2-%C3, and the products of Q_net × %Cities, Q_net × GDP, and Q_net × population density. We further found dominant contribution of lignin to the total number of assigned formulas for the samples collected from the channels in the Huxi watershed and the central lake using high resolution mass spectroscopy. We conclude that Q_net is of key importance for the optical properties of CDOM molecules in the various regions of Lake Taihu and the connecting channels.

In-depth molecular characterization and biodegradability of water-extractable organic nitrogen in Erhai Lake sediment

Dissolved organic nitrogen (DON) constitutes a significant fraction of the total dissolved nitrogen content of most aquatic systems and is thus a major nitrogen source for bacteria and phytoplankton. The present work applied Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) to a compound-level analysis of the depth-dependent molecular composition of water-extractable organic nitrogen (WEON) in lake sediment. The study focused on Erhai Lake, China. It was found that a large portion (from 16.33 ± 7.87 to 39.54 ± 5.77%) of the WEON in the lake sediment was reactive under cultivation by algal or bacteria. The WEON in the mid-region of Erhai sediment particularly exhibited a lower bioavailability, having been less affected by the basin environment. The FT-ICR MS results revealed the presence of thousands of compounds in the Erhai Lake sediment samples collected at different depths, with the N-containing compounds accounting for 28.3-34.4% of all the compounds. The WEON molecular weight was also observed to increase with increasing sediment depth. A van Krevelen diagram showed that the lignin-type components were dominant (~ 56.2%) in the sediment WEON, contributing to its stabilization and reducing the risk of sediment nutrient release. The FT-ICR MS results further revealed 204 overlapping formulas of WEON for each core sediment sample, attributable to the presence of refractory components. It was observed that 78.4% of the formulas were within the lignin-like region, suggesting unique allochthonous DON sources. The aliphatic component proportion of all the unique formulas was also found to increase with increasing sediment depth. This indicates that, with the development and evolution of the Erhai Basin, the more labile WEON components were transformed into more stable lignin-like substrates, with a positive effect on the Lake Erhai ecosystem. Graphical abstract ᅟ.

Investigating binding characteristics of cadmium and copper to DOM derived from compost and rice straw using EEM-PARAFAC combined with two-dimensional FTIR correlation analyses

The binding of Cd and Cu to dissolved organic matter (DOM) derived from compost (CP) and rice straw (ST) was studied through an approach combining parallel factor (PARAFAC) analysis and two-dimensional correlation spectroscopy (2DCOS). Both humic-like and protein-like substances have been observed in CP and ST. Comparing with the Cu, Cd exhibited a lower affinity to DOM, and the quenching curve of Cd failed to be fitted by the Ryan and Weber Model, indicating that the environmental risk caused by applying CP or ST to Cd-polluted soil should be considered. The stability constants (log K_M) of Cu ranged from 3.87 to 5.03, and a larger value was found in CP than those in ST. Protein-like component also showed obvious fluorescence quenching with heavy metals addition. Besides, in CP, phenol-OH and carboxyl showed the fastest response to Cd and Cu, respectively; however, in ST, amide group and phenol-OH displayed the fastest response to Cd and Cu, respectively. These results provide a more detailed knowledge of the interaction mechanism of heavy metals with DOM at the molecular level, which is of great significance for reasonable application of compost and rice straw and efficient control of heavy metal in farmland soil.

Production of soluble microbial products in aerobic granular sludge system under the stress of toxic 4-chlorophenol

The purpose of this study was to evaluate the effect of toxic 4-chlorophenol (4-CP) on the production of soluble microbial products (SMP) in an aerobic granular sludge system. Compared with the control experiment, [Formula: see text] removal efficiency generally reduced by 37.90% after the exposure of 4-CP of 10 mg L^-1, indicating that 4-CP had a toxic effect on the performance of biological nitrification. Meanwhile, the contents of polysaccharides and proteins (PN) of SMP increased from 31.0 ± 0.32 and 0.8 ± 0.12 mg L^-1 to 39.0 ± 0.56 and 61.70 ± 0.78 mg L^-1, respectively. Specific oxygen uptake rate (SOUR) showed that the values of (SOUR)_H, [Formula: see text]and [Formula: see text] reduced by 27%, 48% and 41% in the presence of 4-CP. The characteristic and chemical components of SMP were assessed by using three-dimensional excitation-emission matrix (3D-EEM), synchronous fluorescence spectra, two-dimensional correlation spectroscopy (2D-COS), scanning electron microscope and Fourier transform infrared spectroscopy . 3D-EEM implied that SMP were mainly composed of tryptophan PN-like, aromatic PN-like and humic acid-like substances after 4-CP exposure. Synchronous fluorescence spectra coupled with 2D-COS suggested that SMP samples changed in the following sequences: PN-like substances > humic acid-like substances in the absence of 4-CP, and shorter wavelength > longer wavelength in PN-like substances in the presence of 4-CP.

Composition and copper binding properties of aquatic fulvic acids in eutrophic Taihu Lake, China

Fulvic acid (FA) plays a significant role in biogenic-elemental cycling in aquatic ecosystems which is highly dependent on their organic composition. In this study, the aquatic FA contents and binding properties during bloom and non-bloom periods in Taihu Lake were investigated by two-dimensional correlation spectroscopy Fourier transform infrared spectroscopy (2D-COS-FTIR), nuclear magnetic resonance (NMR) and elemental analysis. Compared with non-bloom FA, bloom FA was of lower nitrogen content and higher C/N ratio. It contained more carboxylic and aliphatic groups while less amide groups. 2D-COS-FTIR spectra evidenced the carboxyl groups in bloom FA had the fastest response to Cu(II) binding. Also, polysaccharide in bloom FA was more susceptive to Cu(II) concentrations than that in non-bloom FA. While comparing with bloom FA, the N-rich organic compounds in non-bloom FA exhibited faster binding sequence with Cu(II). A comprehensive scheme about the interaction process of FA-Cu(II) showed that both nitrogenous and oxygenic groups in FAs were active in binding to Cu(II). The alteration in binding behaviors of organic groups in FAs to Cu(II) may have been driven by algal products and microbial community variety in Taihu Lake. Our results here have the potential to contribute significantly to future studies of dissolved organic matter dynamic biogeochemistry processes and trace metal cycling processes in eutrophic lakes.

Heavy-metal pollution alters dissolved organic matter released by bloom-forming Microcystis aeruginosa

The risk of heavy metals to aquatic ecosystems was paid much attention in recent years, however, the knowledge on effects of heavy metals on dissolved organic matter (DOM) released byMicrocystiswas quite poor, especially in eutrophic lakes.

Molecular characterization of lake sediment WEON by Fourier transform ion cyclotron resonance mass spectrometry and its environmental implications

The compositional properties of water-extractable organic nitrogen (WEON) affect its behavior in lake ecosystems. This work is the first comprehensive study using Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) for the characterization of the molecular composition of WEON in lake sediment. In sediments of Erhai Lake in China, this study found complex WEON species, with N-containing compounds in the northern, central, and southern regions contributing 34.47%, 42.44%, and 40.6%, respectively, of total compounds. Additionally, a van Krevelen diagram revealed that lignin components were dominant in sediment WEON structures (68% of the total), suggesting terrestrial sources. Furthermore, this study applied ESI-FT-ICR-MS to the examination of the environmental processes of lake sediment WEON on a molecular level. The results indicated that sediment depth impacted WEON composition and geochemical processes. Compared with other ecosystems, the double bond equivalent (DBE) value was apparently lower in Erhai sediment, indicating the presence of relatively fewer and smaller aromatic compounds. In addition, the presence of a large number of N-containing species and abundant oxidized nitrogen functional compounds that were likely to biodegrade may have further increased the potential releasing risk of WEON from Erhai sediment under certain environmental conditions.